Allow for using rustls instead of native-tls

[xfbs]

Should restson maybe have parameters to allow for using rustls?

[spietika]

Hyper supports rustls so it should not be a big effort to add the support here, and I think such option parameter would make sense.

For now, it should already be possible (I haven't tested this) to use rustls by constructing your own Hyper client and using the with_client() in the builder: https://github.com/spietika/restson-rust/blob/master/src/lib.rs#L210

Though this is not very convenient so having it as an option would make sense.

[spietika]

This is now (finally) possible in version 1.4.0 by using rustls feature.